1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly to an image forming apparatus in which a light beam is modulated and a recording material such as photosensitive material is exposed by being scanned with the light beam. The present invention also relates to an image forming apparatus in which images are recorded by reciprocating scanning.
2. Description of the Related Art
In a digital laboratory system or the like for recording digital image data on photographic printing paper, an image exposure apparatus is provided in which scan-exposure is carried out by using a light source (for example, a laser device) which generates a light beam (laser beam). For example, a general image exposure apparatus for photographic printing paper includes a light source which generates light beams of red (R), green (G) and blue (B) to obtain a color image, and based on color image data, the light beam is modulated for each of the colors R, G and B.
While the light beams are each deflected by a deflector such as a polygon mirror in a main scanning direction and photographic printing paper is being conveyed in a sub-scanning (slow scanning) direction, the photographic printing paper is exposed by scanning via a scanning optical system including a transmission type optical system such as an fθ lens or a cylindrical lens, thereby allowing a color image to be recorded thereon. Further, in this image exposure apparatus, light beams of R, G and B are modulated by using a large-size and expensive acousto-optic modulator (AOM) or the like to form an image. That is, the scanning optical system was designed or light beams were modulated so that a plurality of light beams have equal characteristics on the photographic printing paper.
However, when an equal rotational angle-type deflector such as a polygon mirror is used, the polygon mirror which is a rotator becomes large and the deflector itself needed to be made larger. Further, the light beams of R, G and B must be transmitted through a transmission type optical system, characteristics relating to color such as chromatic aberration must be constantly taken into consideration in the design for a lens. Moreover, it is necessary to built-in an external modulator for modulating light beams of R, G and B. Therefore, in order to obtain an image of good image quality, the image exposure apparatus needed to be made larger and a complicated structure thereof was required.
Also, for example, in an image recording apparatus, there are many cases in which laser light is caused to scan only in one direction by using a polygon mirror to compensate an optical face tangle error correction or the like, and thereafter, a recording material is moved in a sub-scanning direction while a main scanning line is being generated by the laser light.
In scanning light in a reciprocating manner by means of a reciprocating deflector such as a galvanometer scanner, when a recording material is moved in the sub-scanning direction at a fixed speed, directions to which the main-scanning lines are inclined differ from one another depending on a scanning direction. Therefore, it is difficult to generate main scanning lines in parallel.
Specifically, when laser light having an amplitude W is caused to scan in a reciprocating manner in the main scanning direction U so that an image is recorded in the range of a recording width B (see FIG. 13), the recording material is moved in the sub-scanning direction at a fixed speed. As a result, the main scanning line R obtained by scanning in a direction toward the right side in FIG. 13 and the main scanning line L obtained by scanning in a direction toward the left side are not made parallel to each other with an angle θ formed therebetween, thereby resulting in no formation of a favorable image.
If the spacing between main scanning lines is less than 400 dpi, the aforementioned problem becomes conspicuous. Even if the spacing is greater than 400 dpi, when a photosensitive material is used as the recording material, density unevenness is apt to become conspicuous depending on properties of photosensitive material or the size of spot light.
Accordingly, even when an image recording apparatus having an optical system which allows reciprocating scanning, such as a galvanometer scanner, is used, the main scanning line R or main scanning line L is generated by one-way scanning so as to realize recording of a high-quality image in the existing circumstances.